"They won't and we won't--though we could. A bank of those new million
watt tubes--perhaps a hundred of them--and we'd have a pretty effective
heater--but an awful waste of power. I've got something better."

"New?"

"Somewhat. I've found out how to make the mirror field in a plate of
metal, instead of a block. Come on to the lab, and I'll show you."

"What's the advantage? Oh--weight saved, and silver metal saved."

"A lot more than that, Mac. Watch."

* * * * *

At the laboratory, the new apparatus looked immensely lighter and
simpler than the old. The atostor, the ionizer, and the twin
ion-projectors were as before, great, rigid, metal structures that would
maintain the meeting point of the ions with inflexible exactitude under
any acceleration strains. But now, instead of the heavy silver block in
which a mirror was figured, the mirror consisted of a polished silver
plate, parabolic to be sure, but little more than a half-inch in
thickness. It was mounted in a framework of complex, stout metal braces.

Kendall started the ion-flame at low intensity, so the UV beam was
little more than a spotlight.

"You missed the point, Mac. Now--watch that tungsten-beryllium plate.
I'll hold the power steady. It's an eighteen-inch beam--and now the
energy is just sufficient to heat that tungsten plate to bright red.
But--"

Kendall turned over a small rheostat control--and abruptly the
eighteen-inch diameter spot on the tungsten-beryllium plate began
contracting; it contracted till it was a blazing, sparkling spot of
molten incandescence less than an inch across!

"That's the advantage of focus. At this distance of a few hundred feet
with a small beam I can do that. With a twenty-foot beam, I can get a
two-foot spot at a distance of nearly ten miles! That means that the
receiving end will have the pleasure of handling _one hundred times the
energy concentration_. That would punch a hole through most anything.
All you have to do is focus it. The trouble being, if it's out of focus
the advantage is more than lost. So if there's any question about
getting the focus, we'll get along without it."

"A real help, if you do. That would punch a hole before the Stranger
ship could turn away as they do now."

Kendall nodded. "That's what I was after. It is mainly for the forts,
though. We'll have to signal the dope to the Mars Center and Deenmor
stations. They can fix it up, themselves. In the me